This invention relates to a hot press used, for example, for producing a thin printed board (substrate) and a multilayered board such as a liquid crystal glass board bonded by a low-viscosity adhesive, and more particularly to a hot press suitable for forming a board requiring a uniform bonding pressure.
Generally, a printed board is produced as follows. A predetermined number of printed circuit boards, having respective predetermined circuit patterns printed thereon, and a predetermined number of prepregs, each formed by impregnating a cloth substrate with an adhesive resin, are alternately stacked, and these materials (the stacked printed circuit boards and prepregs) of the printed board are heated and pressed by heat plates. Then, simultaneously with a temperature rise, the adhesive resin is brought from a softened, fluidized condition into a lowest viscosity condition. Thereafter, as chemical reactions proceed, the adhesive resin is brought into a cured, stable condition. As a result, the plurality of printed circuit boards are integrally bonded together by the adhesive resin of the prepregs to produce a multilayered printed board.
In order to eliminate voids between the layers of the printed board and also to achieve uniform-thickness forming so as to achieve a high-density construction of the printed board, there has been proposed a method in which the process is carried out in a vacuum atmosphere until the adhesive resin of the prepregs is softened and fluidized, and then during a curing stage following the fluidization, the process is carried out in an atmosphere of high-pressure gas.
For example, Japanese Patent Unexamined Publication No. 3-128195 discloses a method in which, using heat plates for heating blanks of a multilayered board (substrate) as well as upper and lower bolsters for pressing the blanks of the multilayered board through these heat plates, heat and pressure are applied uniformly to the multilayered board blanks, and space between the upper and lower bolsters is enclosed by a sealing means, and the interior of this sealing means is kept at a vacuum (subatmospheric pressure) until the adhesive resin of the prepregs is softened and fluidized, and then the interior of the sealing means is kept to a high-pressure (air pressure) atmosphere during a curing stage following the fluidization of the adhesive resin. By this method, the pressure distribution is made uniform at the time of adhesively bonding the blanks of the multilayered board.
Japanese Patent Unexamined Publication No. 59-215299 discloses a method in which, with respect to the configuration of pressing surfaces, buffer plates, provided at heat plates, are changed into a configuration, meeting with bonding conditions, by the use of auxiliary rams in bolsters, so that a bonding operation can be effected with a proper surface pressure distribution.
Japanese Patent Unexamined Publication Nos. 61-43543 and 61-43565 disclose a uniform-pressure forming method in which a high-pressure vessel called an "autoclave" is used, blanks of a multilayered board are covered with a film or a sheet, the interior of this cover film or sheet is reduced in pressure, and then the multilayered board blanks are heated and pressed within the high-pressure vessel, using N.sub.2 gas or CO.sub.2 gas.
In order to achieve high-precision uniform-thickness forming by the uniform pressure in the above board bonding process, an attempt has been made to effect the pressing with an improved surface precision of the press. However, the improvement of the surface precision is limited, and besides the board blanks to be bonded together have themselves ruggedness, undulations, and so on, and therefore there has been a limit to the uniform-pressure pressing by the surface pressing.
Particularly when bonding thin glass board blanks together, a low-viscosity adhesive is used under a low pressure so as to prevent breakage of the glass board blanks. However, under such a low pressure, uniform pressing cannot be effected because the glass board blanks have undulations and so on, which has resulted in a problem that the thickness of the formed products has varied.
To produce a board of a special inner layer construction or a board of the type, such as a metal board and a glass board, in which adhesives of different properties are distributed therein (for example, a board in which a low-viscosity adhesive is used generally over the entire area thereof while a high-viscosity adhesive is used locally), there are occasions when a high-precision uniform-thickness forming can be achieved using local pressing in addition to uniform-pressure pressing.
However, the conventional bonding operation for the board has depended mainly on the application of pressure to the viscous fluid as described above, and has been directed to the application of a uniform pressure to this fluid. In those boards in which a large amount of high-rigidity substances are present in the direction of application of the pressure, while the viscous fluid is present in a very small amount in this direction, the precision of the board thickness is lowered in the vicinity of the high-rigidity substances under the influence of these high-rigidity substances in the board. The precision of the board thickness may be enhanced by locally increasing the pressure of those portions containing the high-rigidity substances; however, with a press it has heretofore been impossible to locally increase the pressure, although the pressure distribution can be gently varied.